mesa: Support BindBuffer{Base,Offset,Range} with a buffer of 0.

mesa: Support BindBuffer{Base,Offset,Range} with a buffer of 0.

_mesa_lookup_bufferobj returns NULL for 0, which caused us to say
"there's no such buffer object" and raise an error, rather than
correctly binding the shared NullBufferObj.

Now you can unbind your buffers.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>